Preparation of ZrO2 on flat,conducting SiO2/Si(100) model supports by wet chemical techniques; X-ray photoelectron spectroscopy and Auger depth profiling

Model supports consisting of a thin layer of SiO₂ on a silicon single crystal have been used to study ZrO₂/SiO₂/Si model catalysts made by wet chemical preparation methods. Auger depth profiling and angle-dependent X-ray photoelectron spectroscopy show that catalysts prepared by a surface reaction between zirconium ethoxide and hydroxyl groups on the SiO₂ contain a highly dispersed zirconium phase that is converted to ZrO₂ upon calcination.     

Surface reactions of nitrogen oxide and ethylene on rhodium (111)

Temperature programming of NO and C₂H₂ coadsorbed on Rh(111) gives rise to the desorption of a number of gases. Where H₂, H₂O, CO₂ and N₂ are the main products at low C₂H₂ coverages, significant amounts of HCN, CO and NO evolve at higher C₂H₄ coverages. Static SIMS indicates the formation of a large supply of adsorbed CN species, part of which desorbs as HCN, while the remainder decomposes and is responsible for delayed formation of N₂. For the highest C₂H₄ coverages the majority of the initially adsorbed NO desorbs as HCN.     

TiO2-Supported Mo Model Catalysts: Ti as Promoter for Thiophene HDS?

The combination of thiophene hydrodesulfurization (HDS) activity measurements and X-ray photoelectron spectroscopy on flat model systems of sulfided HDS Mo catalysts showed that sulfided Ti-species can act as a promoter in the same way as Co and Ni, although less effectively. This explains the higher thiophene HDS activity and hydrogenation selectivity of Mo/TiO₂ compared with Mo/Al₂O₃, while for Ni-promoted Mo catalysts the difference between the two supports is negligible.     

Intrinsic Thiophene Hydrodesulfurization Kinetics of a Sulfided NiMo/SiO2 Model Catalyst: Volcano-Type Behavior

A realistic planar model of a silica-supported NiMo sulfide hydrotreating catalyst was used to study the intrinsic kinetics of thiophene hydrodesulfurization over a broad temperature range at atmospheric pressure. The specific nonporous nature of the model catalysts excludes possible diffusion limitations, which are commonly encountered in kinetic studies of their industrial porous analogues. The unique possibility to measure the intrinsic chemical kinetics over a large temperature range allowed us to observe a Volcano-type behavior for thiophene hydrodesulfurization. The specific kinetic parameters are discussed in terms of possible mechanisms.     

Mars-van Krevelen-like Mechanism of CO Hydrogenation on an Iron Carbide Surface

Computational chemistry is used to explore a mechanism for CO hydrogenation to methane on iron carbides. As CO dissociation is endothermic on carbon terminated Fe₅C₂ (100) cuts, we explore a path starting with the hydrogenation of the surface, which liberates iron 4-fold sites for adsorption and dissociation of CO. The reaction cycle to methane resembles the Mars-van Krevelen mechanism for oxidation reactions.     

CO bonding and hydrogenation activity of promoted noble metal catalysts during restructuring in high-pressure CO hydrogenation: FeIr/SiO2

Bimetallic FeIr/SiO₂ catalysts that are active for methanol production from synthesis gas (CO+H₂) bind CO less strongly, and exhibit higher activity for the hydrogenation of ethylene in the presence of CO, than catalysts that produce mainly methane. It is argued that promotion of the noble metal serves to weaken the adsorption of CO, thereby lowering its tendency to dissociate, and, most importantly, enhancing the surface coverage of hydrogen. Both factors are favorable for methanol formation. This revised version was published online in July 2006 with corrections to the Cover Date.     

In situ Mössbauer spectroscopy of carbon-supported iron catalysts at cryogenic temperatures and in external magnetic fields

A highly dispersed carbon-supported iron catalyst has been studied within situ Mössbauer spectroscopy at temperatures down to 5 K and with external magnetic fields. It is shown that measurements of spectra in the presence of large magnetic fields considerably improves the information obtained from Mössbauer spectra.     

Preface

Topics in Catalysis -     

Planar model system for olefin polymerization: the Phillips CrO x /SiO2 catalyst

A planar CrO _x /SiO₂/Si(100) model for the Phillips ethylene polymerization catalyst has been prepared by spincoat impregnation from an aqueous solution of CrO₃. The model catalyst polymerizes ethylene from the gas phase at 160°C with a constant activity and forms a 400 nm thick layer of polyethylene in 1 h. The superior definition of the polymer films produced on this catalyst and the control over the distribution of active sites on its flat surface make this model catalyst an ideal substrate for kinetic studies on catalytic polymerization and for morphologic studies of the polymer product by scanning force microscopy. At extremely low catalyst loading we observe isolated polymer islands formed on single chromium sites. The work also opens attractive opportunities for future studies of nascent morphology of catalytically formed polymers. This revised version was published online in June 2006 with corrections to the Cover Date.     

Preface

Topics in Catalysis -